1. Field of the Invention
The invention relates to a method of reconstructing the spatial current distribution in a biological object, at least one component of the magnetic field produced by the current sources being measured at a number of points outside the object, after which the current distribution at the volume elements situated within the object is reconstructed on the basis of the measuring values, and also relates to a device for performing the method.
A method and a device of this kind are known from the publication "SQUIDs und Bilder neuronaler Strome" by O.Dossel and W. Kullmann (Phys. B1. 44 (1988) No. 11, pp. 423-425).
2. Description of the Prior Art
Therein, for the reconstruction, the magnitude and direction of the current densities in the individual volume elements constituting the three-dimensional object region must be calculated from the magnetic fields measured outside the object region to be examined. It can be demonstrated that this so-called inverse three-dimensional problem cannot be unambiguously solved. The known method, therefore, uses reconstruction algorithms which enable an approximative calculation of the current distribution. The distribution thus determined, however, deviates from the actual distribution.
It is to be noted that from the publication by J. W. H. Meijs et al. "The EEG and MEG, using a model of eccentric spheres to describe the head", IEEE Trans. Biomed. Eng., Vol. BME-34, pp. 913-920, 1987, it is already known to derive information regarding the individual head or brain geometry from magnetic resonance tomogram or computer tomograms, thus enabling more accurate determination of the position of a single current dipole. This known method aims to determine, using the values of the magnetic flux density measured at different measuring points, the position of a single tangential, point shaped current dipole in a volume conductor, i.e. so that the measured magnetic field corresponds as well as possible to the magnetic field which would be measured if the current dipole were present at the relevant area. This approach utilises magnetic resonance tomograms or computer tomograms which represent the morphology of the head and in which areas of similar electrical conductivity are marked so as to enable more accurate modelling of the volume currents in the brain.
It is an object of the present invention to provide a method of the kind set forth so that such deviations are reduced. This object is achieved in accordance with the invention in that in a representation which contains the morphological structure of the object the surfaces on which the current sources are presumably present are specified, the reconstruction being limited to the volume elements which are situated on the surfaces.
The invention is based on the recognition of the fact that for many brain activities it is known that the neuronal current sources are situated on given surfaces. For example, tumors can initiate epileptic attacks because of their space requirements. The interior of the tumor is electrically inactive, the epileptical focus (current source) is situated somewhere on the periphery of the tumor. For some forms of focal epilepsy which cannot be traced to a tumor, a morphologically modified region occurs; the focus pursued is very probably present on the periphery thereof. Finally, it is known that evoked fields which can be measured on the head after a stimulation of the sense organs originate from a spatially definable zone of the folded cerebral cortex.
Consequently, the reconstruction of the current source density takes place only for the specified (two dimensional) surfaces and not for a three dimensional region within the object to be examined. Consequently, the two dimensional inverse problem occurs which can in principle be unambiguously solved. Therefore, the reconstruction in practice is influenced merely by the fact that the measurement of the magnetic field is performed at a finite number of points and that measurement thus takes place with a finite accuracy.
A device for performing the method in accordance with the invention is characterized in that it comprises a measuring device for determining the magnetic flux density outside the object, a memory for storing the measuring values thus obtained, a unit for determining the volume elements which are situated on surfaces to be specified, and a reconstruction unit for determining the current distribution the volume elements from the values stored.